Our overall aim is to describe the contribution of maternal RNA stored in the mammalian egg to embryogenesis, in particular the role of maternal ribosomes and messenger RNA's in early protein synthesis and the possible role of maternal messages in differentiation. Our analysis depends on the method devised for labeling the RNA of ovulated ova by exposure of growing oocytes to tritiated nucleosides in vivo. Our immediate objectives are to continue the characterization of the RNA of the mouse egg by its distribution on sucrose gradents. We will also continue the description of ribonucleoprotein particles in which messenger RNA's are stored in the egg, as well as further definition of an unusual lability in the structure of egg ribosomes. Next it is proposed to measure the progressive loss of total maternal RNA in the developing embryo. Once optimal conditions for labeling embryos have been defined, an attempt will be made to trace the activation of stored maternal ribosomes and messenger RNA's and their lifetime in the embryo. Finally we plan to examine the structure of chromosomes in growing mouse oocytes and the synthesis of RNA during oocyte growth.